Valve actuators are used to open or close valves. They are often pneumatic or hydraulic, and use a cylinder and piston arrangement to move a control rod. The control rod will be attached to the gate of a gate valve, and by filling or emptying the piston with an appropriate working fluid, the gate may be moved and the valve opened or closed.
A typical design will comprise a piston deployed within a chamber, the piston being attached to a control rod. The piston will define two chambers within the actual chamber: a first chamber and a second chamber. The first chamber will be subjected to the working fluid acting upon the piston, and will include a working fluid inlet. To enable the piston and control rod to return to a certain position, be it open or closed, the second chamber will include some form of resilient bias, such as a spring. As the working fluid is pumped into the first chamber under pressure, the fluid will act on the piston, expanding the first chamber, and compressing the second chamber and the spring. The movement of the piston will in turn move the control rod and gate, thus moving the valve typically from a closed to an open position. Once the fluid pressure is released, the force stored in the spring will act upon the piston, re-expanding the second chamber and compressing the first chamber, thus typically moving the valve from an open to a closed position.
When the pressure is vented, the valve is preferably arranged such that the compressed spring drives the valve to the closed position. This valve set-up is often used as part of a ‘fail safe’ system and in some cases such as when actuators are used to operate X Tree production master valves, a combination of the actuator spring closing force and the valve gate are used to cut wire or logging cable, so that if the valve is required to close in an emergency, it can do so even during times when work over operations are under way. A great deal of force is required, not only to overcome the valve drag but also to cut cable.
The inventor of the present invention has recognised that a major driver in the design of such actuators is the spring requirement and often the size of an actuator is dictated by the size of the closing spring. Space is a constraint in many processing facilities, be it in subsea manifolds or in petro chemical processing facilities. Weight is also an issue, particularly where deployment of hardware involves lifting large packages of equipment into deep water.
A disadvantage of such an arrangement is that the chamber must be a considerable length in order to house the spring and allow for the necessary movement of the piston, leading to a large and cumbersome actuator.
More importantly, conventional actuators suffer from a major disadvantage that the closing force supplied by the resilient element/spring directly to the moving actuator is not constant, because the spring effectively supplies more closing force at the beginning of the closing stroke than at the end, but it is typically at the end of the stroke that a great deal of force is required to finish the closing stroke, particularly to cut the cable if present.